The lattice parameters of the bee (beta) and omega (Omega) phases occurring metastably in a series of Zr-rich Zr-Nb alloys have been determined at and above room temperature (T-R) using neutron diffraction techniques. In the first place, the effect of temperature changes upon the lattice parameters of the beta and Omega phases in alloys with 10 and 18 at.% Nb was monitored using neutron thermodiffraction. A method of analysis is applied to the data which involves a confrontation between the observed structural properties and an idealised -or "reference"- behaviour (RB) which admits a simple mathematical description. A generalised form of Vegard's law is adopted as RB for the beta phase, whereas a specific RB is proposed for the Omega structure. The experimental data are well accounted for by this interpretation scheme, leading to a picture of the isothermal reactions occurring at high temperature which involves the transfer of Nb from the Omega to the beta phase. Finally, the neutron diffraction data on the Omega phase are combined with an electron microscopy study for the alloy with 10 at.% Nb aged at 773 K, which provides information on the composition of this phase and its evolution towards thermodynamic equilibrium.